San Clemente

This. I don’t want to spoil the surprise, but it’s not just the visit of a church : it’s a time travel.

There are already good suggestions, I'd add Ostia antica or San Paolo fuori le Mura

Musei Capitolini? Centrale Montemartini? Villa Borghese? La Farnesina? Galleria Doria Pamphili? Santa Maria Maggiore? San Giovanni in Laterano? Foro Boario? Museo nazionale romano? Terme di Caracalla? Santa Sabina? Terme di Diocleziano and Santa Maria degli Angeli? Villa Giulia and the National Etruscan Museum? Catacombs of Saint Callixtus?

Go outside the beaten path, take the train for a day trip somewhere.

Did you visit Castel Sant'Angelo?

Bike tour on Via Appia

Santa Maria in Aracoeli. One of the finest churches in Rome. It's near Campidoglio.

Case Romane del Celio, the Aula Gotica, Catacombs, Scala Sancta

Baths of Caracalla (get the audio guide) - we were blown away by them

ok you've already seen the boring parts! (/s) let's move into MAXXI and/or MACRO for some contemporary art 🤓🤓🤓 ooorrrr skip the night and visit all over again the boring parts by night so that the temperature is not high and there are sooo fewer people, then at 6.30AM have a well deserved breakfast